.sctn. 1.1 Field of the Invention
The present invention concerns methods and apparatus for generating a plasma torch. More specifically, the present invention concerns a module for generating a plasma torch and a system, using an array of such modules, for generating a plasma torch. In addition, the present invention also concerns applications for the plasma torch module and the plasma torch generation system.
.sctn. 1.2 Related Art
The present invention relates generally to atmospheric pressure plasma generation devices (or "plasma sources"). Atmospheric pressure plasma sources do not need vacuum systems in their setups. They can have open structures, thereby allowing plasma to be exposed directly to the open air. An atmospheric plasma source may be used for reflecting or absorbing microwave radiation (See, e.g., the article: R. J. Vidmar, "On the Use of Atmospheric Pressure Plasmas as Electromagnetic Reflectors and Absorbers," IEEE Trans. Plasma Sci., Vol. 18, pp. 733-741 (August 1990), hereafter referred to as "the Vidmar article".), processing materials (See, e.g., the article: M. I. Boulos, "Thermal Plasma Processing," IEEE Trans. Plasma Sci., Vol. 19, pp. 1078-1089 (December 1991), hereafter referred to as "the Boulos article".), sterilizing (See, e.g., the articles: K. Kelly et al., "Room Temperature Sterilization of Surfaces and Fabrics with a One Atmosphere Uniform Glow Discharge Plasma," Journal of Industrial Microbiol Biotechnology, Vol. 20, pp. 69-74 (1998), hereafter referred to as "the Kelly article"; and E. Garate et al., "Atmospheric Plasma Induced Sterilization and Chemical Neutralization," IEEE Int. Conf. Plasma Sci., Vol. 98CH36221, p. 183 (1998), hereafter referred to as "the Garate article".), and neutralizing chemicals (See, e.g., the Garate article). Most of these applications require the plasma to be dense and have a large volume.
Using AC or DC glow discharges to produce atmospheric pressure plasmas generally results in low densities (e.g., less than or equal to 10.sup.11 electrons/cm.sup.3) and a small volume. (See, e.g., the article: J. R. Roth, "Industrial Plasma Engineering--Vol. 1, Briston: Principles Institute of Physics (1995), hereafter referred to as "the Roth article".) Although the volume of the plasma can be increased by using microwave (See, e.g., the article J. E. Brandenburg et al., "Experimental Investigation of Large Volume PIA Plasmas at Atmospheric Pressure," IEEE Trans. Plasma Sci., Vol. 26, pp. 145-149 (April 1998), hereafter referred to as "the Brandenburg article".) or photon ionization processes, the plasma density is still low.
Operation of a discharge in the high current, diffused are, mode can be achieved by introducing gas flow in the discharge to form a plasma torch. (See, e.g., the article: T. B. Reed, "Induction-coupled Plasma Torch", J. Appl. Phys., Vol. 32, No. 5, pp. 821-824 (1961), hereafter referred to as "the Reed article".) The inertia of the gas flow carries the plasma to give a longer discharge current path without significantly affecting the electric field distribution between the electrodes.
The discharge can be either a high frequency inductive (See, e.g., the article: M. I. Boulos, "Thermal Plasma Processing," Pure Appl. Chem., Vol. 57, No. 9, pp. 1321-1357 (1985) hereafter referred to as "the second Boulos article".) or low frequency capacitive (See, e.g., the article: E. Koretzky et al, "Characterization of an Atmospheric Pressure Plasma Generated by a Plasma Torch Array," Phys. Plasmas, Vol. 5, No. 10, pp. 3774-3780 (1998), hereafter referred to as "the Koretzky article".). The inductive torch has the feature of being electrode-less, but a high current power supply is used to induce a large electric field in the gas for its breakdown. Consequently, the structure of an inductive torch is relatively large and is therefore unsuitable for certain applications. On the other hand, the capacitive torch employs a pair of electrodes to apply the high voltage, provided directly by the power supply, to the gas for a discharge. A power transformer can easily step up the voltage to the required level. This difference in the discharge field distributions (i.e., localized near the electrodes versus that distributed along the coil) causes the operating temperature of the capacitive torch to be much lower than that of the inductive torch. (See, e.g., the article S. P. Kuo et al, "Temperature Measurement of an Atmospheric Pressure Plasma Torch," Rev. Sci. Instrum., Vol. 70, No. 7 (1999), hereafter referred to as "the Kuo article".)
In view of the foregoing deficiencies of known plasma torches, there is a need for a plasma source that can be used independently, or with other sources to produce a relative dense plasma having a relatively large volume. Such plasma sources should be easily replaceable and serviceable. Finally, such a plasma source should be relatively easy and fast to start.